Applications involving fiber optic communications systems typically utilize light waves having wavelengths in the near infrared (0.8 to 3.0 micrometers in wavelength). These systems presently represent the greatest usage for near infrared detectors. However, other applications such as temperature sensing, night vision, eye-safe range finding, process control, lidar, and wind-shear detection require detectors with higher sensitivity and faster response times in the near IR region. Recently, InGaAs detectors have been investigated for light detection at wavelengths greater than 1.65 .mu.m because of their potential for high performance and reliability. Such detectors have demonstrated high quantum efficiencies (&gt;70%), low dark current (&lt;100 mA cm.sup.2 at -5 V), and rise times less than one nanosecond at room temperature. Other materials (Ge, PbS, InSb, PtSi, HgCdTe, etc.) have been used for detectors at wavelengths greater than 2 .mu.m, but they generally have to be cooled to low temperatures, often have very slow response, or have high dark currents.
Since In.sub.0.53 Ga.sub.0.47 As detects light at wavelengths =1.65 .mu.m, in order to detect longer wavelengths more indium must be added to the ternary compound, thereby decreasing the bandgap. In this case, the lattice parameter can no longer match that of the InP substrates. A graded layer technique has been developed to accommodate the lattice mismatch between the substrate (a.sub.o =5.869 .ANG.) and the In.sub.x Ga.sub.1-x As (x &gt;0.53) absorption layer (a.sub.o &gt;5.869 .ANG.). In this technique, InAs.sub.y P.sub.1-y (y.ltoreq.1) buffer layers with increasing lattice parameters are grown in between the InP substrate and the absorption layer. This prevents lattice mismatch dislocations from propagating from layer to layer, enabling the growth of absorption layers with good optoelectronic properties. It is also possible to use In.sub.x Ga.sub.1-x As as grading layers by increasing the indium concentration. However, by using InAsP which has a larger bandgap than In.sub.x Ga.sub.1-x As, better spectral response is obtained for back illumination (light enters through the substrate). Also, using larger bandgap materials as buffer layers results in detectors with lower dark current. InGaAs detectors with up to 2.6 .mu.m cutoff wavelength using InAsP graded layers have been successfully fabricated with good opto-electronic properties.